Electrical conductivity (or its inverse, resistivity) is an important property of subsurface formations in geological surveys and prospecting for oil, gas, and water because many minerals, and more particularly hydrocarbons, are less conductive than common sedimentary rocks. Thus, a measure of the conductivity is often a guide to the presence and amount of oil, gas, or water. Induction logging methods are based on the principle that varying electric currents, due to their associated changing magnetic flux, induce electric currents.
Propagation logging instruments generally use multiple longitudinally-spaced transmitter antennas operating at one or more frequencies and a plurality of longitudinally spaced receiver pairs. An EM wave is propagated from the transmitter antenna into the formation in the vicinity of the borehole and is detected at the receiver antenna(s). A plurality of parameters of interest can be determined by combining the basic measurements of phase and amplitude. Such parameters included the resistivity, dielectric constant and porosity of the formation as well as, for example, the degree to which the fluid within the borehole migrates into the earth formation.
The transmitter antennas on induction logging instruments generate a time-varying magnetic filed when a time-varying electric current is applied to them. The time-varying magnetic field induces eddy currents in the surrounding earth formation. The eddy currents induced voltage signals in the receiver antennas, which are then measured. The magnitude of the induced voltage signals varies in accordance with the formation properties. In this manner, the formation properties can be determined.
Conventional antennas consist of coils mounted on the instruments with their axes parallel to the central or longitudinal axis of the instrument. Therefore, the induced magnetic field is also parallel to the central axis of the well and the corresponding induced eddy currents make up loops lying in planes perpendicular to the well axis. Traditionally, induction tools use copper wires to form axial coils for both transmitters, receivers and bucking coils. Bucking or balancing coils are used in some designs to lessen the effect of cross-coupling between the transmitter and receiver coils. Some designs have implemented bucking coils which are combined with either the transmitter or receiver coils in a co-wound configuration.
A portion of the induction tool with axial coils in the direction of tool axis is shown in FIG. 1a. Bobbins 10 and spacers 12 are mounted on the mandrel 14 to form the array stack of the induction tool. Sleeve 16 is then slid over the entire antenna structure. The copper wire 18 is wound on the outside diameter of the bobbin 10 to form either transmitter or receiver coils. The direction of the moment of the coils is the same of that of the tool axis. The center of the moment is indicated as orthogonal to the tool axis. The bobbin 10 is usually made of ceramics.
The winding process is very critical to the precision of the magnetic moment of the coil. But current winding process is very technique-sensitive and labor-intensive. Any human errors will have significant effects on the accuracy of the coils. Therefore, this process is not only unreliable, but also very expensive.
The response of the described induction logging instruments, when analyzing stratified earth formations, strongly depends on the conductive layers parallel to the eddy currents. Nonconductive layers located within the conductive layers will not contribute substantially to the response signal and therefore their contributions will be masked by the response of the conductive layers. Accordingly, the nonconductive layers are not detected by typical logging instruments.
Solutions have been proposed to detect nonconductive layers located within conductive layers. U.S. Pat. No. 5,781,436 describes a method that consists of selectively passing an alternating current through transmitter coils inserted into the well with at least one coil having its axis oriented differently from the axis orientation of the other transmitter or receiver coils.
The coil arrangement shown in U.S. Pat. No. 5,781,436 consists of several transmitter coils with their centers distributed at different locations along the instrument and with their axes in different orientations. Several coils have the usual orientation, i.e. axis parallel to the instrument axis, and therefore the well axis. Other coils have an axis perpendicular to the instrument axis. This latter arrangement is usually referred as a transverse coil configuration.
Thus, transverse EM logging techniques use antennas having a magnetic moment that is transverse to the longitudinal axis of the well. The magnetic moment m of a coil or solenoid-type antenna is represented as a vector quantity oriented parallel to the induced magnetic field, with its magnitude proportional to the corresponding magnetic flux. In a first approximation, a coil with a magnetic moment m can be seen as a dipole antenna due to the induced magnetic poles.
In some applications it is desirable for a plurality of magnetic moments to have a common intersection with different orientations. For example, dipole antennas could be arranged such that their magnetic moments point along mutually orthogonal directions. An arrangement of a plurality of dipole antennas where in the dipole magnetic moments are oriented orthogonally in three different directions is referred as a triaxial orthogonal set of magnetic dipole antennas.
A logging instrument equipped with an orthogonal set of magnetic dipole antennas offers advantages over an arrangement that uses standard solenoid coils distributed at different axial positions along the instrument having axes in different orientations, such as proposed in U.S. Pat. No. 5,781,436.
However, it is not convenient to build orthogonal magnetic dipole antennas with conventional solenoid coils due to the relatively small diameters required for logging instruments. Arrangement consisting of solenoid coils having axes perpendicular to the central axis of the well occupy a considerable amount of space within the logging instrument.
Flexible circuit boards have been contemplated for application in a multi-axial antenna design. Specifically, copper traces are mounted on a flexible printed circuit board made of an insulating material to allow the coil or set of coils to be placed on-top-of a set of underlying copper wire wound axial coil. The transverse saddle coils of the flexible printed circuit board contain four planar rectangular or circular coils of N turns separated from the wire wound axial coil by the insulating material of the circuit board. When formed to a non-conducting cylinder, the two pairs of planar traces are associated to generate two transverse coils, one in each the x- and y-direction. The underlying wire wound coil is an axial coil situated in the z-direction of the triaxial antenna configuration. These flexible circuit transverse coils have been integral to designing a co-located antenna tool, but do not address the challenges associated with wire-wound axial coils. This is an entirely different approach from the way that induction coils have been built for the past 50 years.